Heh, Oblivion got Patrick Stewart to do voice acting, among many other notables; the sorts of people who do it for a profession. I haven't heard the plane-shift voice acting, and usually forget it exists, since I can't seem to find the option that turns it on.
I doubt anyone would ever use such a system for role-playing, unless they've managed to make their voice just like they want. Even then, it would be incongruous and jarring when one player out of 40 randomly starts using it, when others can not or will not. It would also be a few steps harder to implement than the pre-recorded NPC chat.
If you want to talk to people during the game, use one of the clients out there made for it. They'll probably work better than an integrated one ever would.